The operation of certain solar energy heating systems requires that discrete temperature measurements be made of a solar collector, a heat storage unit, and a space to be heated in order to determine the routing of heat. Most such systems employ copper-constantan thermocouples, one being used to measure a discrete one of the system parameters. The low level output of a thermocouple must be amplified, and, in prior systems, the output of individual thermocouples was individually amplified, and this has required that the amplifier have a low input offset drift and that a fixed offset be removed with either a potentiometer or a resistor before the system is placed in operation. Each amplified temperature measurement is then referenced to a thermal standard which is either an oven or a diode voltage generator. Solar equations must then be solved using two or more temperature measurements summed in a separate amplifier.
An alternate system uses thermistors, each being connected in a bridge to drive a discrete amplifier and the solar equations solved by using two or more outputs from the separate amplifiers, which are then summed in another amplifier. Such a system requires an extremely accurate reference voltage plus an extremely accurate thermistor to make each measurement.
The two systems discussed have several disadvantages. First, both systems deal with absolute temperatures when actually the difference temperatures are ultimately needed. Further, a difference temperature is accurately obtained only when the absolute temperature of each measurement is accurately made. This is not easily accomplished since accuracy is degraded by amplifier offsets and gain error as well as sensor errors. To get around these problems, high quality and thus expensive components must be used.